He was the son of David.
The book of the generation of Jesus Christ, Jesus Christ, the son of David. Matthew 1:1
He was the seed of David, the fruit of his loins his according to the flesh.
Therefore being a prophet [David], and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne. Acts 2:30
Of this man's seed [David's] hath God, according to his promise, raised unto Israel a Saviour, Jesus. Acts 13:23
Concerning his son Jesus Christ our Lord, which was made of the seed of David according to the flesh. Romans 1:3
Remember that Jesus Christ of the seed of David. 2 Timothy 2:8
I Jesus have sent mine angel to testify unto you these things in the churches. I am the root and the offspring of David. Revelation 22:16
(How could David call him "Lord" if he was his son?)
The beginning of the gospel of Jesus Christ, the Son of God. Mark 1:1
Saying, What think ye of Christ? whose son is he? They say unto him, The son of David. He saith unto them, How then doth David in spirit call him Lord, saying, The LORD said unto my Lord, Sit thou on my right hand, till I make thine enemies thy footstool? If David then call him Lord, how is he his son? Matthew 22:45
And Jesus answered and said, while he taught in the temple, How say the scribes that Christ is the son of David? For David himself said by the Holy Ghost, The LORD said to my Lord, Sit thou on my right hand, till I make thine enemies thy footstool. David therefore himself calleth him Lord; and whence is he then his son? Mark 12:35-37